The present invention relates to a sustained-release pheromone dispenser or, more particularly, to a pheromone dispenser capable of sustainedly emitting a sex pheromone compound used for population control of an insectan pest to the environment at a constant rate over a long period of time.
Many proposals have been made and are under practice in the prior art for sustainedly emitting a sex pheromone compound and the like effective for population control of a pest as contained in a suitable container to the environment at a constant rate of emission over a length of time. These prior art proposals can be classified into several types. The first type proposals are for a pheromone dispenser in which the sex pheromone compound is supported as being adsorbed by a suitable adsorbent as a carrier or as being admixed with a polymeric resin forming a carrier layer. For example, U.S. Pat. No. 4,017,030 discloses a pheromone dispenser in which the sex pheromone compound is contained in a capillary tube having an open end and emitted from the open end of the tube. Pheromone dispensers of this type have a defect in respect of the usually very short serviceable life because a single dispenser can contain only an extremely small amount of the pheromone compound. U.S. Pat. No. 4,160,335 discloses a pheromone dispenser having a multilayered laminate of polymeric films to serve as a carrier of the pheromone compound with an object of controlling the rate of pheromone emission from a pheromone-carrying layer of a polymeric resin admixed with the pheromone compound. Pheromone dispensers of this type have a serious disadvantage that, while the rate of pheromone emission is sometimes too large in the initial stage of exposure thereof to the open field, the rate gradually decreases in the lapse of time and, moreover, a considerable amount of the pheromone compound initially contained in the dispenser remains unreleased out of the dispenser to cause a large loss of the expensive compound.
Further, West German Patent No. 2,832,248 discloses a dispenser of (S)-cis-verbenol and methyl butenol as an attractant of wood borers, i.e. Limonoria lignorum, in which the attractant compounds are absorbed by a porous carrier and wrapped with a film of a plastic resin such as polyethylene and the like. West German Patent No. 2,945,655 discloses a dispenser of the same attractant in which the chemicals are mixed with a pasty base and a plastic bag is filled with the mixture. Japanese Patent Kokai 59-13701 and U.S. Pat. No. 4,445,641 describe a pheromone dispenser in which the pheromone compound is absorbed by a porous carrier and the pheromone-holding carrier is coated with a coating film of polyethylene and the like to control the emission rate of the pheromone compound through the coating film. These pheromone dispensers have a disadvantage in common that a considerable proportion of the pheromone compound absorbed in the carrier remains in the carrier unreleased even after prolonged exposure to the atmosphere to cause a large loss of the pheromone compound.
As is understood from the above discussion, important characteristics required for a pheromone dispenser are that the pheromone compound contained in the dispenser is emitted to the ambience at a constant rate over a long period of field exposure and that the pheromone compound initially contained in the dispenser is released out of the dispenser as completely as possible so as to minimize the loss of the expensive pheromone compound remaining as unutilized. These requirements can be satisfied by a dispenser in which a liquid pheromone compound is contained and sealed in a container having a wall provided with a barrier layer capable of exhibiting an adequate permeability to the liquid compound contained therein.
The prior art pheromone dispensers belonging to the second type of the proposals are further classified into three classes including so-called microcapsules containing the pheromone compound as the core material disclosed in U.S. Pat. No. 2,800,457, No. 2,800,458 and No. 3,577,515 and elsewhere, capillary tubes of polyethylene containing the liquid pheromone compound and sealed at the ends disclosed in Japanese Patent Kokai 56-142202, 57-9705, 57-45101, 57-72904, 57-156403, 59-216802 and 60-215367 and elsewhere, and those in which the pheromone compound is wrapped in a sheet or bag of a plastic film disclosed in Journal of Economic Entomology, volume 62, No. 2, pages 517-518 (1969).
The microcapsules of the above mentioned first class are not so widely utilized in practice because of the high costs for the preparation, large loss of the pheromone compound in the process of encapsulation into microcapsules and too large rate of pheromone emission and short serviceable life as a consequence of the extremely large surface area of the microcapsules. In respect of the capillary tube-type dispensers as the second class, various improvements have been attempted by using a polyethylene tube of adequate wall thickness or by combining the polyethylene tube with a metal wire side-by-side to facilitate the hanging works on the tree twigs. Though advantageous in respect of the sustained releasability and relatively long life for service, it is reported in Journal of Economic Entomology, volume 75, No. 6, pages 1431-1436 (1985) that these tubular dispensers have a defect that the rate of pheromone emission is subject to a considerable decrease in the latter half stage of the life on service. The journal article recited for the plastic sheet- or bag-wrapped pheromone dispenser describes an example in which (Z)-7-dodecenyl acetate, which is a sex pheromone of cabbage loopers, is enclosed and sealed together with sand in a bag of 3 cm by 5 cm wide made of a polyethylene film having a thickness of 50 to 150 .mu.m and is used for the purpose of intercommunication disruption. Although uniformity is obtained in the rate of pheromone emission, pheromone dispensers of this type have a disadvantage of a short life of service for only about two weeks or so because polyethylene works only insufficiently as a barrier against permeation of the pheromone compound.
Dispensers of the third type in the prior art are proposed in U.S. Pat. No. 3,343,663, No. 3,785,556 and No. 4,161,283 and elsewhere in which a biologically active vaporizable substance other than sex pheromones of insects and the like, is wrapped with a sheet or a bag made of a plastic film and the compound is emitted to the atmosphere at a controlled rate through the plastic film.
Although a variety of different types of pheromone dispensers have been proposed as is described above, those by using a capillary tube made of polyethylene and by using a plastic film or bag are the only examples of the pheromone dispensers used for the purpose of intercommunication disruption or population control of pests each containing at least 40 mg of a sex pheromone compound which is an unsaturated aliphatic ester, aldehyde or ketone having 12 to 20 carbon atoms in a molecule. These dispensers are all made by using polyethylene as a material of the dispenser body. This is because this polymeric material has an adequate affinity with pheromone compounds but is inert thereto absorbing only a very small amount thereof, has excellent workability such as good weldability in the sealing works of the dispenser body containing the pheromone compound and has excellent mechanical strengths and resistance against water as the essential properties required for pheromone dispensers.
The tubular dispensers have a defect that, although the rate of pheromone emission can be controlled over two months or longer as a consequence of the wall thickness of at least 300 .mu.m of the capillary tube of polyethylene, the rate of pheromone emission is subject to a considerable decrease in the latter half of the life for service. The bag-like dispensers are under a limitation in respect of the workability of a plastic film into bags that the plastic film must have a thickness not exceeding 200 .mu.m so that bag-type dispensers made from polyethylene alone have a limited life of service and are not suitable for the purpose of emission control over two months or longer as in the tubular dispensers because polyethylene cannot provide a sufficient barrier. When the temperature of ambience is high and the wind velocity is low, in addition, the vaporization velocity of the pheromone compound from the dispenser walls would be too low to dissipate the whole amount of the pheromone compound having reached the outer surface of the dispenser after permeation through the polyethylene walls so that the outer surface the dispenser is always wettish with the pheromone compound oozing out to increase deposition of dusts with great troubles in the outdoor use of the dispensers.
As is described above, there is known no high-performance pheromone dispenser in which a plastic film having a thickness of 200 .mu.m or smaller serves as a barrier against permeation of the pheromone compound enabling control of the rate of pheromone emission over a period of at least two months without a wettish condition of the surface. The inventors have continued extensive investigations to develop a sustained-release dispenser of a sex pheromone compound of insects which is an aliphatic unsaturated acetate, aldehyde or ketone compound, of which the amount of the pheromone compound contained in a single dispenser is at least 40 mg, and arrived at a conclusion that a sustained-release pheromone dispenser could well satisfy the requirements without the problems and disadvantages in the tubular or bag-like dispensers in the prior art.